Summary: This paper details experience with corrosion in the annuli of wells in the Aran field, north Sumatra, Indonesia. Wells in this field characteristically have high production temperatures, rates, and pressures, and the produced fluids contain the potentially corrosive constituents CO2 and H2S. The field has been in production for about 10 years, and a number of wells have been worked over once or even twice. These workovers have detected corrosion problems that have led to changes in completion practices and have focused attention on methods to monitor and to detect ongoing annular corrosion. It is normal practice to fill the annulus between the last cemented casing in a well and the production tubing with a noncorrosive "packer" fluid. Various types of packer fluid have been used in the Arun wells, and some of these fluids have caused unforeseen corrosion problems, which led to workovers to replace the problem fluid.